1. Field of the Invention
The present invention generally relates to a photo (light) amplifying apparatus, and more particularly, to a photo amplifying apparatus for amplifying wavelength division multiplexing (WDM) signals that includes a measuring unit that measures the power of a specific wavelength, a measuring unit that measures the total power of all wavelengths, and a controlling unit that controls the output power of the photo amplifying apparatus.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is a pervading broad band technology in which a plurality of photo signals of different wavelengths is transmitted via a single optical transmission path.
The wavelength division multiplexing technology is applied to multi-channel optical communications of various distances. In practice, wavelength division multiplexing communication systems are required to flexibly support optical communications of various wavelengths and distances.
In general, a WDM photo amplifying apparatus of the WDM communication system amplifies photo signals so that the output power of output photo signals of each wavelength becomes equal. To achieve this object, the WDM photo amplifying apparatus controls excitation light sources and optical attenuators provided therein based on information about the power of photo signals monitored at various stages in the optical path and information for setting gain, the number of wavelengths, and output power, for example, received from an exterior resource.
FIG. 1 is a schematic diagram showing a conventional photo amplifying apparatus. The photo amplifying apparatus shown in FIG. 1 includes photo amplifying units 1, 2, a variable optical attenuator 6, a control unit 7, optical splitters 12-15, and photo diodes (optical monitors) 32-35.
The photo amplifying units 1, 2 further include Erbium-doped Fiber Amplifiers (EDFA) 3, 4, laser diodes (excitation light sources) 21, 22, and wavelength mixers 41, 42, respectively. The photo amplifying unit 1 further includes a gain equalizer 5.
The EDFAs 3, 4 are excited by the laser diodes 21, 22, respectively.
FIG. 2 is a graph showing the distribution of power (dBm) over wavelength (nm) of wavelength division multiplexing signals. As shown in FIG. 2, Xin, denotes the power of photo signals of each wavelength λ1-λn. The WDM signal having the power distribution shown in FIG. 2 is input to the photo amplifying apparatus via a input terminal IN. A fraction of the input photo signals is split from the main signal by the photo splitter (photo coupler) 12, and converted into an electric signal by the photo diode 32.
FIG. 3 is a schematic diagram showing the power of the photo signals monitored by the photo diode 32. The output of the photo diode 32 depends only on the total power (=Xin+10×log(n)) of photo signals over all wavelengths λ1-λn. Likewise, the outputs of the photo diodes 33-35 depend only on the total power of photo signals over all wavelengths λ1-λn incoming to the photo splitters 13-15, respectively.
The control unit 7 controls the power of each stage based on the total power of the photo signals over all wavelengths λ1-λn monitored by the photo diodes 32-35. As a result, the photo amplifying apparatus shown in FIG. 1 outputs the objective output photo signals via an output terminal OUT.
FIG. 4 is a schematic diagram for explaining the automatic level control (ALC) performed by the control unit 7. The control unit 7 of the photo amplifying apparatus shown in FIG. 1 automatically controls the optical level of each stage as follows:                (1) Controlling the laser diode (excitation light source) 21 so that the difference between the output PPD33 of the photo diode 33 and the output PPD32 of the photo diode 32 becomes constant.PPD33−PPD32=A(constant)  (1)        (2) Controlling the laser diode (excitation light source) 22 so that the difference between the output PPD35 of the photo diode 35 and the output PPD34 of the photo diode 34 becomes constant.PPD35−PPD34=B(constant)  (2)        
In the case that the photo amplifying unit 1 (or 2) cannot achieve a predetermined gain A (or B), the control unit 7 raises the gain of the photo amplifying unit 2 (or 1) so as to make A+B constant.
(3) Controlling the variable attenuator 6 so that the output power from the output terminal OUT reaches the objective output level.
As a result of the above automatic gain control, an output of which level is determined based on the number of wavelengths is obtained via the output terminal OUT.
FIGS. 5A and 5B are schematic diagrams for explaining a problem that occurs in the case wherein the control unit 7 fails to control the output from the output terminal OUT at the level determined based on the number of wavelengths.
FIG. 5A illustrates the case wherein two wavelengths are used. The output powers of both wavelengths λ1, λ2 are assumed to be equal to the same value Xin. Under such a condition, the case in which the light source of the wavelength λ2 accidentally malfunctions is examined below.
Since the number of wavelengths to be amplified by the photo amplifying apparatus is reduced from two wavelengths to one wavelength, the output level of the photo amplifying apparatus is expected to be halved. However, if the control unit 7 is not informed that the number of input wavelengths is reduced to one, the control unit 7 compensates for the reduction in power level caused by the disappearance of wavelength λ2 by increasing the output level of λ1. Consequently, the control unit 7 cannot keep the output level per wavelength constant.
To solve this problem, in a large-capacity long-distance WDM communication system, an upper-rank photo transmitting apparatus informs the photo amplifying apparatus of a lower-rank photo transmitting apparatus of information such as the number of wavelengths and the output level of each wavelength to configure the photo amplifying apparatus of the lower rank photo transmitting apparatus.
The control unit 7 of the photo amplifying apparatus of the lower-rank photo transmitting apparatus controls the output based on the information (configuration information) received from the upper-rank photo transmitting apparatus so that the output power of the photo amplifying apparatus is kept at the level corresponding to the number of wavelengths.
Since the properties of optical parts built in conventional photo amplifying apparatuses are dispersed, the conventional photo amplifying apparatuses are adjusted one by one in an adjustment process of production so that the gradient of gain over wavelength becomes flat and the gains satisfy the following equations:PPD33−PPD32=A(dB)(constant), and  (1)PPD35−PPD34=B(dB)(constant)  (2)
Japanese Patent Laid-open Application No. 2000-312046 discloses a technique with which difference between gains of a photo amplifying apparatus determined by various input wavelengths caused by change in optical fiber loss can be eliminated. According to this application, a controlling unit built into the photo amplifying apparatus controls the operation of the photo amplifying apparatus based on the intensity of received reference light.
Japanese Patent Laid-open Application No. 2000-196169 discloses a technique for a photo amplifying apparatus of a WDM communication system that multiplexes and transmits a plurality of photo signals with which the degradation of nonlinear property and signal-to-noise ratio due to change in the number of channels is eliminated. According to this application, a photo amplifying apparatus is provided with the following: a first portion including an optical fiber with rare earth element doped, an excitation laser diode, and an automatic optical gain control circuit; a second portion including an optical attenuator and an automatic level control circuit; and a monitor signal processing circuit. Before a change in the number of channels, the photo amplifying apparatus outputs amplifying a WDM signal of which level is determined by the number of channels. When the photo amplifying apparatus is informed of the change in the number of channels, the photo amplifying apparatus temporarily fixes the transmissivity of the optical attenuator, and keeps its gain constant so as to output a photo signal of which level corresponds to the number of channels. After the change in the number of channels, the photo amplifying apparatus resumes controlling the transmissivity of the optical attenuator and its gain.
In the case in which a signal of a certain wavelength unexpectedly vanishes due to the malfunction of a light source, for example, the upper-rank photo transmission apparatus determines that the number of wavelengths has changed. After the determination, the upper-rank photo transmission apparatus informs the photo amplifying apparatus of a lower-rank photo transmission apparatus receiving configuration information (the number of wavelengths and the output levels, for example) from the upper-rank photo transmission apparatus, about the change in the number of wavelengths. Accordingly, the photo amplifying apparatus receives the information indicating that the number of wavelengths has changed in a delay (from hundreds of msec to several sec) after the number of wavelengths actually changes.
A conventional photo amplifying apparatus absorbs the delay to the extent from hundreds of msec to several sec in the following manner.
An optical attenuator with a dead time from hundreds of msec to several sec may be used; the optical attenuator operates with the delay from hundreds of msec to several sec. Even if the photo amplifying apparatus receives the information indicating that the number of wavelengths has changed, during the delay (from hundreds of msec to several sec) after the number of wavelengths actually changes, the optical attenuator does not react. Accordingly, the photo amplifying apparatus is not affected by the delay time.
Alternatively, monitoring circuits sampling signal levels at each stage may be required to retain their sampled values for a retention time of about several seconds. Since an optical attenuator is controlled based on the sampled signal levels monitored by the monitoring circuits, the optical attenuator is controlled after the retention time of several seconds. Accordingly, the photo amplifying apparatus can avoid being affected by the delay in receiving the information.
However, the time required for the information of the number of wavelengths to be transmitted from the upper-rank photo transmission apparatus to the photo amplifying apparatus depends on the performance of the upper-rank photo transmission apparatus for determining the number of wavelengths, and consequently, differs one after another. There remains a problem in that the above dead time and/or holding time needs to be optimized for each upper rank photo transmission apparatus.
Because the total gain of a photo amplifying apparatus is desired to be indifferent for various wavelengths, the gain of the photo amplifying unit of the photo amplifying apparatus needs to be adjusted to compensate for the difference in optical efficiency of the remaining portion of the photo amplifying apparatus other than the photo amplifying unit.
Conventionally, the gain of the photo amplifying unit of each WDM photo amplifying apparatus is individually adjusted in the production process based on a test condition (for example the maximum number of wavelengths). Accordingly, if the WDM photo amplifying apparatus is used in practice under a different condition (for example the number of wavelengths) from the test condition defining the maximum number of wavelengths, for example, the desired flatness of the total gain of the WDM photo amplifying apparatus may not be achievable.
For example, as shown in FIG. 6, even though the photo amplifying apparatus is adjusted in its production process based on the maximum number of wavelengths (ten, for example), the photo amplifying apparatus is often used with the number of wavelengths (seven, for example) being less than the maximum number. In this case, the gain of the photo amplifying apparatus may exhibit a gradient as shown in FIG. 7.
The invention taught in Japanese Patent Laid-open Application No. 2000-312046 still inherits a problem that the wavelength of the reference light needs to be different from the wavelength of the main signals. Another problem of the invention is that the number of wavelengths cannot be identified.
According to the invention taught in Japanese Patent Laid-open Application No. 2000-196169, when the wavelength number is changed, the transmissivity, for example, of the optical attenuator is temporarily fixed, and then, after the wavelength number is changed, the control of the optical attenuator is resumed. This invention also inherits the above problems.